1. Field of the Invention
Embodiments of the invention generally relate to a method and apparatus for local polishing control in an electrochemical mechanical polishing system.
2. Background of the Related Art
Electrochemical mechanical polishing generally removes material from a semiconductor substrate through an electrochemical/chemical or a combined electrochemical/chemical and mechanical process. In one example of an electrochemical mechanical polishing system, a substrate or wafer is retained on a substrate support in a feature side up orientation. A polishing head having a conductive polishing pad and an internal counter electrode is placed in contact with the feature side of the substrate. The polishing head and the substrate are moved relative to one another in a predefined polishing motion. An electrolytic polishing fluid is disposed on the substrate and provides a conductive path between the substrate and the counter electrode. The substrate is electrically biased through the conductive pad relative to the counter electrode to drive a dissolution reaction at the substrate's surface to polish the substrate.
Copper is one material that may be polished using electrochemical mechanical polishing. Typically, copper is polished utilizing a two step process. In the first step, bulk of the copper is removed, typically leaving some copper residue projecting above the substrate's surface. The copper residue is then removed in a second or over-polishing step.
However, the removal of copper residue may result in dishing of copper features below the plane of surrounding material, typically an oxide or other barrier layer. The amount of dishing typically is related to polishing chemistries and processing parameter utilized in the over polish step, along with the width of the copper features subjected to polishing. As the copper layer does not have a uniform thickness across the substrate, it is difficult to removes all the copper residue without causing dishing over some features while not removing all of the copper residue over others. Thus, it would be advantageous if some areas of copper may be selectively polished while not polishing other areas to yield complete copper residue removal and minimized dishing.
Therefore, there is a need for a method and apparatus for local polishing control in an electrochemical mechanical polishing system.